1. Field of the Invention
The present invention relates to an electronic component connecting method, and more particularly, to an electronic component connecting method suitable for use in mounting a chip-type electronic component, such as an IC chip, on a wiring board.
2. Description of the Related Art
FIGS. 2A to 2F show a conventional electronic component connecting method disclosed in Japanese Unexamined Patent Publication No. 5-190599. This connecting method results in the state shown in FIG. 2F, in which an IC chip 1 serving as a first electronic component is mounted on a wiring board 2 serving as a second electronic component.
First, as shown in FIG. 2A, an IC wafer 3 is prepared as a mother component which is later cut (FIG. 2C) to form a plurality of the IC chips 1. On one main surface 4 of the IC wafer 3, connecting electrodes 5 are formed at predetermined positions in regions corresponding to the IC chips 1.
Three connecting electrodes 5 are shown in FIG. 2A. On the right-side connecting electrode 5, a projecting electrode 6 is formed to serve as a bump electrode. FIG. 2A sequentially illustrates the process of forming this projecting electrode 6 in connection with the left-side connecting electrode 5, the center connecting electrode 5, and the right-side connecting electrode 5.
The projecting electrode 6 is basically formed by ball bonding. First, as illustrated in connection with the left-side connecting electrode 5 in FIG. 2A, a ball 9 is formed at the leading end of a gold wire 8 that passes through a capillary 7. Next, the ball 9 is placed on the connecting electrode 5, as illustrated in connection with the center connecting electrode 5, and is pressed against the connecting electrode 5 by the capillary 7. Then, the gold wire 8 is cut off the ball 9, as illustrated in connection with the right-side connecting electrode 5 in FIG. 2A. In this way, the projecting electrode 6 is given a two-stage structure by the ball 9 and a part of the gold wire 8.
By repeating the above-described steps, projecting electrodes 6 that serve as bump electrodes are formed on all the connecting electrodes 5 on the main surface 4 of the IC wafer 3, as shown in FIG. 2B.
Subsequently, as shown in FIG. 2C, the IC wafer 3 is cut into a plurality of IC chips 1 by, for example, dicing.
Then, a conductive adhesive 10 is applied on the projecting electrodes 6 formed on the connecting electrodes 5 of the IC chip 1 by, for example, dipping, as shown in FIG. 2D.
Also shown in FIG. 2D is the preparation of the wiring board 2. On one main surface 11 of the wiring board 2, connecting electrodes 12 are formed corresponding to the connecting electrodes 5 on the IC chip 1.
Next, the IC chip 1 is pressed against the wiring board 2, as shown in FIG. 2E, while the main surface 4 of the IC chip 1 and the main surface 11 of the wiring board 2 are opposed to each other, as shown in FIG. 2D. In this case, since the conductive adhesive 10 is not hardened, it deforms to follow the surfaces of the connecting electrodes 12 on the wiring board 2, and is brought into close contact with the projecting electrodes 6 and the connecting electrodes 12. Furthermore, since the projecting electrodes 6 have a two-stage structure, spaces are formed between the projecting electrodes 6 and the connecting electrodes 12 so as to receive the conductive adhesive 10. For this reason, the conductive adhesive 10 does not generally spread out in a horizontal direction.
Subsequently, the conductive adhesive 10 is hardened while the IC chip 1 and the wiring board 2 are aligned with each other, as shown in FIG. 2E.
Next, as shown in FIG. 2F, a filling material 13 is applied and hardened between the IC chip 1 and the wiring board 2. The filling material 13 serves to reinforce the connected portion between the IC chip 1 and the wiring board 2, to relax thermal stress, and to improve moisture resistance of the IC chip 1, and is filled in a clearance between the IC chip 1 and the wiring board 2.
In this way, according to the conventional electronic component connecting method shown in FIGS. 2A to 2F, since the projecting electrodes 6 have a two-stage structure, spaces are formed between the projecting electrodes 6 and the connecting electrodes 12 so as to receive the conductive adhesive 10, as described above. Therefore, even when the IC chip 1 is pressed against the wiring board 2, the conductive adhesive 10 is prevented from being horizontally spread out. This makes it possible to reduce the pitch between a plurality of connecting electrodes 5 and the pitch between a plurality of connecting electrodes 12, to reduce the size of the IC chip 1, and to improve packaging density.
The electronic component connecting method shown in FIGS. 2A to 2F has, however, the following problems which must be solved.
First, a plurality of steps shown in FIG. 2A must be performed to form a projecting electrode 6 having a two-stage structure, and be repeated for each of the connecting electrodes 5. Therefore, much time is taken to form projecting electrodes 6 on all the connecting electrodes 5, and the length of time is proportional to the number of the connecting electrodes 5. In particular, the time to process becomes longer as the arrangement density of the connecting electrodes 5 increases. This results in an increase in cost.
In the above-described ball bonding for forming the projecting electrodes 6, metal-to-metal solid phase bonding is performed by applying load and ultrasonic energy, and this may cause damage to the IC wafer 3, and further, to the IC chip 1.
In order to reduce such damage, an active area, where elements such as a capacitor and a transistor are arranged, must not be placed directly below the connecting electrodes 5. This countermeasure, however, produces another problem in that the size of the IC chip 1 is increased to form the connecting electrodes 5 outside the active area.